1. Field of the Invention
The invention relates to a stud welding process in which a stud to be welded to a surface is pressed against the surface with an acceleration force and a welding current is triggered wherein a positioning pressure is applied to at least two points spaced away from the stud axis.
2. Description of Related Art
In principle, two forms of stud-welding processes are known. One employs a tip which is simultaneously subjected to a current and applied under force to the part to be welded on. This is referred to as tip ignition method. Here, large currents are built up and then discharged via the tip. Owing to the short time available during the discharge, it is not possible to accelerate the stud even further toward the part to be welded on and to immerses it deeper into the melt. As a result, this process cannot be used in the case of a stud diameter over 8 mm, as is known from specialist books (cf. Betterman "Bolzenchweisstechnik" [Stud-welding technique], page 4).
Thus, for example, JP-A-56-91 983 describes an apparatus for stud-welding, in which a guide device ensures the vertical guidance of a welding gun in relation to a surface onto which the stud is welded. The stud itself is pushed into a guide block which in turn is displaceable, by means of springs coordinated with it, in the axial direction of the welding gun. During the welding process, the stud is pressed against the surface by means of the welding gun. The guide block is provided for ensuring that the welding gun remains positioned at right angles to the surface during the welding process.
Although relatively large diameters can also be welded by means of the elevator ignition technique, this method requires an inert gas atmosphere, the energy requirement is higher, the welding time is longer and the tendency to burn through the stud on the part to be welded on is greater so that at least very strong welding discolorations always remain behind on the lower surface of the plate when such a stud is welded onto a thin plate. However, this is undesirable in many cases.
Consequently, it has not been possible to date to weld on studs having a diameter of more than 8 mm
cleanly, PA0 without an inert gas and PA0 in a short time.
This gives rise to an object of the present invention permit in particular a tang to be welded onto a trowel blade cleanly, without an inert gas and in a short period of time.
Extensive attempts have been made to achieve this object. An initial shown that an important phenomenon leading to the difficulties mentioned is the fact that, when the welding current is triggered, the stud has the tendency to "float away" on the end which has become liquid. This tendency cannot be counteracted by the conventional pressure-application devices acting exclusively axially, i.e. the investigations led to the discovery that this is not just a centering problem during the welding process, which may last a very short time where the tip ignition process functions within milliseconds.
Based on this discovery, according to a preferred embodiment of the invention, a positioning pressure is applied to at least two points spaced away from the axis of the stud. By virtue of the fact that the pressure now involves force components which are parallel and symmetrical in terms of the angle, decentering is not so easily possible, and any decentering tendencies due to the geometry of the stud or of the device can preferably be compensated by separately adjusting the pressure at the at least two points. For this purpose, all that is required is a test weld in order to be able to make the necessary adjustment, "separate" adjustment being intended to mean one in which an adjustment of one pressure does not necessarily result in an equally large adjustment of the other pressure.
Although the above statements indicate that the invention can be applied in principle to all embodiments of the stud-welding process, the object of the invention is preferably achieved when the tip ignition method is used.
An application of the process according to the invention in which the advantages are utilized to a particularly high degree is welding a stud to be used as a tang of a trowel onto the trowel surface. In this case, however, it is preferable if the tang is bent prior to welding, since on the one hand this is easier and on the other hand the action of high forces on the welding spot is avoided in this manner.
The centering problem discussed above is even better controlled when a stud having a non-straight shape is held firmly in a mold surrounding it.
Although the process according to the invention has quality advantages for the welding of all stud thicknesses, it can be particularly advantageously used for studs having a cross-sectional thickness (i.e. the diameter in the case of a circular cross-section) greater than 8 mm, for which those skilled in the art previously believed the stud-welding process was in principle unsuitable.
Although it has been found that pressure application to at least two points is entirely sufficient, the application of an additional axial force gives rise to a further improvement.
Another object of the present invention is to provide a device for carrying out stud welding by applying a positioning force to at least two points away from the axis of the stud.
Another object of the present invention is to provide a device wherein at least one pressure member is formed by a fluidic piston/cylinder unit and the stud to be welded is held in a mold.
The stud holder is held exclusively on at least one of the pressure-application devices, but the opening and closing devices for the mold halves of the holder are not rigidly connected to the housing, guidance of the stud during the welding process is determined exclusively by the pressure-application devices. Thus, opening--and if necessary also closing--of the mold halves is effected by means of suitably chosen coupling members which do not prevent the downward movement of the stud in its holder.
Further details of the invention are evident from the following description of embodiments shown schematically in the drawings.